High-pressure Luminescence Properties Of NaErF₄-Based Heterogeneous Core-shell Structure Upconversion Nanoparticles

Heteroepitaxial coating is a common method to enhance the peculiarity of nanocomplex,however a perfectly matched interface is usually not the case which may affect behaviors of the nanocomplex.Although the impact of the resulting interfacial stress is often neglected when analyzing their luminescence spectra,at high pressures the effect can be significantly amplified.In order to unravel the underlying interaction mechanism and also search for high-quality pressure-sensing materials,we have done the following.1.The synthesized NaErF₄@NaLnF₄（Ln = Y,Lu,Gd）and its Tm（3+）-doped derivatives were used as research objects,and the morphology and lattice structure of the nanoparticles were characterized by transmission electron microscopy and X-ray diffraction,and it was found that the nanoparticles of both systems were uniform in size and had good dispersion,and it is also concluded that nanoparticles with tensile stress of NaLuF₄ shell layer have the smallest lattice mismatch and are closer to homogeneous epitaxial growth,exhibiting the strongest upconversion luminescence at atmospheric pressure.2.The fluorescence emission spectra of nanoparticles were tested under high pressure,and it was found that the luminescence intensity of nanoparticles of both systems weakened with increasing pressure,which is due to the fact that the external high pressure shortens the distance between ions and increases the possibility of multiphoton relaxation,cross-relaxation and non-radiative energy transfer,resulting in energy loss,while the high pressure also changes the geometric structure of the crystal and intensifies the heterogeneous core-shell structure of the lattice mismatch,resulting in lattice defects,weakening the luminescence.3.Comparing the red-green light ratio of the luminescence spectra of Tm（3+）-doped and non-doped systems under high pressure,the slope of the red-green light ratio was used to represent the mechanical sensitivity of nanoparticles.It is found that the red-green light ratio of nanoparticles becomes larger with the increase of pressure,which is due to the green light emission from the luminescence center Er（3+） is more sensitive to high pressure,and the green light intensity decays faster than the red light under the effect of external high pressure,which eventually leads to the increase of red-green light ratio.Also found that the mechanical sensitivity of the Tm（3+）-doped system to be greater,because of the unique energy level structure of Tm（3+）,will weaken the green light emission,the formation of near-monochromatic red light emission,while the external high pressure will shorten the distance between the ions,intensify the energy coupling between Er（3+） and Tm（3+）,better play the role of Tm（3+） energy transfer,making the nanoparticle luminescence more sensitive to changes in external pressure.4.In the Tm（3+） doped system,Lu shell layer nanoparticles with tensile stress shows a better mechanical sensitivity,which is due to the ionic radius of Lu（3+） is larger than that of Er（3+）,Lu shell layer needs to generate tensile stress inside to adapt to the core of Er,this force counteracts on the core will generate a compressive force,close the distance between Er（3+） and Tm（3+）,nearly further intensifying the coupling between the ions.In addition,the color coordinate plot shows that the luminescence color of NaErF₄: 0.5% Tm@NaLuF₄ nano changes from orange red to dark red during the high pressure change from atmospheric pressure to about 9 GPa at external pressure,indicating that the luminescence color changes significantly with the pressure change,demonstrating the good mechanical sensing properties of the material.After that,we investigated the stability of NaErF₄:0.5%Tm@NaLuF₄ nanoparticles under high pressure and found that the high pressure did not affect the mechanical sensitivity,luminescence intensity,luminescence peak position,morphology and crystal structure of the nanoparticles,indicating that NaErF₄:0.5%Tm@NaLuF₄ nanoparticles have good stability and are an ideal material for high pressure calibration.